Start slow, and add speed only when you can control them. If your muscles start to contract, you are doing them too fast and/or far. When you get better, speed should be closer to actual speed of your kick, but not 100%.

For dynamic stretching, I've been working with Intu-Flow and Prasara yoga from RMAX ( www.rmaxinternational.com ) and I rarely get stiff or sore, even after heavy workouts. I have also become much more relaxed in my movements as I become able to move smoothly with greater range of motion.

That said, sometimes I will do some extra stretching following a workout, or between workouts. It's fun and feels good when you do it right.